Microbial preparation of metal-substituted magnetite nanoparticles

A microbial process that exploits the ability of iron-reducing microorganisms to produce copious amounts of extra-cellular metal (M)-substituted magnetite nanoparticles using akaganeite and dopants of dissolved form has previously been reported. The objectives of this study were to develop methods for producing M-substituted magnetite nanoparticles with a high rate of metal substitution by biological processes and to identify factors affecting the production of nano-crystals. The thermophilic and psychrotolerant iron-reducing bacteria had the ability to form M-substituted magnetite nano-crystals (MyFe3−yO4) from a doped precursor, mixed-M iron oxyhydroxide, (MxFe1−xOOH, x ≤ 0.5, M is Mn, Zn, Ni, Co and Cr). Within the range of 0.01 ≤ x ≤ 0.3, using the mixed precursor material enabled the microbial synthesis of more heavily substituted magnetite compared to the previous method, in which the precursor was pure akaganeite and the dopants were present as soluble metal salts. The mixed precursor method was especially advantageous in the case of toxic metals such as Cr and Ni. Also this new method increased the production rate and magnetic properties of the product, while improving crystallinity, size control and scalability.